1. Field of the Invention
This invention relates to new rapidly solidified iron base alloys containing aluminum and boron. This invention also relates to the preparation of these materials in the form of rapidly solidified powder and consolidation of these powders (or alternatively the rapidly solidified ribbon-like material) into bulk parts which are suitably heat treated to have desirable properties.
2. Description of the Prior Art
Rapid solidification processing (RSP) techniques offer outstanding prospects of new cost effective engineering materials with superior properties. [See Proc. Int. Conf. On Rapid Solidification Processing; Reston, Va., 1980; Published by Claitors Publishing Division, Baton Rouge, La.]. Metallic glasses, microcrystalline alloys, supersaturated solid solutions and ultrafine grained alloys with highly refined microstructures, in each case, often having complete chemical homogeneity are some of the products that can be made utilizing RSP. [See Rapidly Quenched Metals, 3rd Int. Conf. Vols. 1 and 2, Cantor Ed; The Metals Society, London, 1978].
Several techniques are well established in the state of the art to economically fabricate rapidly solidified alloys (at cooling rates of .about.10.sup.5 to 10.sup.7 .degree. C./sec) as ribbons, filaments, wires, flakes or powders in large quantities. One well known example is melt spin chill casting whereby the metal is spread as a thin layer on a conductive metallic substrate moving at higher speed to form rapidly solidified ribbon. [See Proc. Int. Conf. on Rapid Solidification Processing, Reston, Va., 1977].
The design of alloys made by conventional slow cooling process is largely influenced by the corresponding equilibrium phase diagrams which indicate the existence and coexistence of the phases present in thermodynamic equilibrium. Alloys prepared by such processes are in or at least near equilibrium. The advent of rapid quenching from the melt has enabled material scientists to stray further from the state of equilibrium and has greatly widened the range of new alloys with unique structure and properties availabel for technological application.
Heat and corrosion resistant alloys are capable of sustained operation without corrosion and oxidation when exposed either continuously or intermittently up to or in excess of 1200.degree. F. They find extensive applications in metallurgical furnaces, cement mill equipment, oil refineries, petrochemical furnaces, steel mill equipment, power plant equipment, etc. [refer Source Book on Material Selection, Vol. 2, ASM, p. 39, 1977]. These alloys invariably contain large amounts of chromium and nickel for which the country is dependent on imports to meet its requirement.
Certain drawbacks of the current heat resistant alloys are that they contain substantial amount of chromium which is in short supply because of limited reserves of mineral deposits.
Alloys based primarily on iron and aluminum, relatively inexpensive elements with abundant and more secure reserves, offer excellent alternative possibilities to the current heat resistant alloys containing chromium, nickel and/or cobalt. Iron-aluminum alloys exhibit outstanding oxidation resistance and were considered as potential candidates for a wide variety of heat resistant applications ranging from turbines to components for industrial furnaces. (See E. R. Morgan and V. F. Zackay, Metal Progress, Vol. 68, No. 4, p. 126, 1955 and Journal Iron and Steel Institute, Vol. 130, 1934, p. 389). However, the room temperature brittleness of these alloys has retarded their application. Iron-aluminum alloys containing 13 wt% (24 at %) aluminum, as necessary for adequate high temperature oxidation resistance between 1600.degree. and 2000.degree. F., have poor room temperature mechanical properties, i.e. low tensile strength and poor ductility. It has been suggested that brittleness of Fe-Al alloys with aluminum content generally greater than 13-14 wt% (24-26 at %) is caused in part by the occurrence of ordering. (see W. V. Justusson, V. F. Zackay, and E. R. Morgan, Trans ASM, Vol. 49, pp. 905-923, 1957). Fe-Al-C alloys with 2.1 wt% carbon containing 20.45 wt% aluminum have been reported to have good resistance to internal oxidation upon exposure at 1600.degree. and 1800.degree. F., but these alloys were reported to have negligible ductility and low tensile strength (see J. A. Yater et al, AFS Transactions, Vol. 113, 1976, p. 305).
Recent efforts to obtain improved strength and ductility combined with good corrosion and oxidation resistance in Fe-Al alloys resulted in the development of alloys containing 13 wt% (.about.24 at%) aluminum, 1 to 1.5 wt% titanium and 0.5 to 0.7 wt% boron which were compacted from rapidly solidified powders (see E. R. Slaughter et al, Report AFML-TR-79-4167, Nov. 1979).
Binary iron-aluminum alloys have a strong susceptibility to microcracking during the ingot casting operation due to low thermal conductivity and high thermal expansivity. This charactteristic feature, coupled with large grain growth at high temperatures, renders hot working iron aluminum alloys difficult. Furthermore, air melting or iron aluminum alloys containing high aluminum contents results in the formation of embrittling grain boundary oxide films which accentuate the tendency for intergranular fracture of the ingots during subsequent hot working operation (see W. Justusson et al, Trans ASM, Vol. 49, pp. 905-923, 1957).
There has been limited efforts, as reported in the prior art, involving the use of RSP techniques to synthesise new iron-aluminum alloys containing &gt;20 at% aluminum with unique chemical composition and microstructures exhibiting superior mechanical properties and corrosion and/or oxidation resistance for numerous industrial/engineering applications.